I agree with this article. The key is each device is more suited for a specific purpose than the other. Combining their capabilities only makes it awkward to use and sub-optimum for the application. However, the thing common among all three is mobility. They all do that well.
Read this article for more : http://www.smartertechnology.com/c/a/Optimized-Systems/Why-Tablets-Dont-Kill-the-Laptop/?kc=STNL09062011STR10